This application is based on application No. 2000-185374 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical disc, and a recording apparatus, a recording method and a reproducing apparatus for the optical disc. More particularly, the present invention relates to a technique for recording primary digital information and secondary digital information onto an optical disc by embedding the secondary digital information into the primary digital information.
2. Background Art
In recent years, optical discs typified by DVDs (Digital Versatile/Video Discs) have been commonly used as media for recording lengthy digital data such as AV (Audio and Visual) data and computer data. For instance, DVD-ROMs which can record upwards of two hours of high-quality moving pictures are already on the market. Techniques for preventing the unauthorized copying of digital contents of recording media like DVDs are required to support secure distribution of the digital contents.
The following description concerns a conventional technique (called a xe2x80x9ccontent encryption methodxe2x80x9d) for preventing such unauthorized copying (see Nikkei Electronics, Nov. 18, 1996, pages 13-14).
The content encryption method is described below with reference to FIG. 1, which shows ordinary recording areas on a DVD. According to this method, a compressed digital content, such as a movie, is encrypted by using secret keys on three hierarchical levels (a title key, a disc key, and a master key), and the encrypted content is recorded into a user information area 9a which can be accessed by a user. The master key is the most important of these keys and is provided only to authorized (licensed) manufacturers of DVD devices. The disc and title keys, which need to be assigned to a DVD and a title, respectively, are encrypted by using the master key and are stored in a control information area (read-in area) 9b which the user cannot access.
The above-described arrangement prevents an unauthorized (unlicensed) DVD reproducing apparatus from decoding encrypted contents. As a result, illegal activities, such as the mass production and retailing of DVDs that store unencrypted digital contents, can be prevented.
Japanese Laid-Open Patent Application No. H11-39721 discloses another technique for preventing the illegal copying of digital contents. According to this technique, copyguard information, which is produced by performing an FM modulation on a reference clock, is confidentially recorded onto an optical disc.
These conventional techniques, however, cannot stop DVD piracy. The unauthorized copying of a DVD can be easily performed, for instance, by bringing two spindle motors into perfect synchronization, placing an original DVD and another DVD on these spindle motors, generating a bit stream by reproducing a data string that is read from the original DVD and synchronizing the data string by using a PLL, and recording the bit stream onto the other DVD as it is.
In this manner, the digital content of a DVD can be copied to another DVD along with the copyguard information or information that is stored in the control information area. The encrypted content of a DVD that is illegally produced in this manner can then be read and decrypted by a licensed DVD device as if it were a genuine disc.
While the above-described techniques prevent the illegal production of DVDs that store unencrypted contents, the illegal activities of when encrypted contents recorded on DVDs are copied to other DVDs as they are cannot be stopped. This means that there is the risk of low-price pirated DVDs which may appear on the market and which may cause extensive commercial damage to the copyright owners of the contents.
The object of the present invention is therefore to provide an optical disc on which digital content is recorded in a manner that prevents the perfect copying of the digital content, and a recording apparatus, a recording method, and a reproducing apparatus for the optical disc.
The stated object is achieved by an optical disc on which primary digital information and secondary digital information are recorded, where the primary digital information is recorded as pits that are formed at discrete reference positions on a track, and the secondary digital information is recorded as deviations of slight amounts in the positions of certain pits from corresponding reference positions.
With this construction, the secondary digital information is embedded into the primary digital information so that the reading of the secondary digital information is difficult. Therefore, even if information on the optical disc is copied to another optical disc according to the presence or absence of the pits, the secondary digital information cannot be copied. This allows the original optical disc to be distinguished from the duplicated optical disc and thereby prevents copyright infringements caused by illegally copying the digital contents of the optical disc.
Here, each slight amount may be within a range where pits on an optical disc, on which only the primary digital information is recorded, deviate from corresponding reference positions during reproduction. This makes it extremely difficult for an ordinary reproducing apparatus to detect the secondary digital information that is hidden within jitter or radial errors. As a result, the dubbing of the secondary digital information together with the primary digital information can be prevented.
Here, the secondary digital information may be recorded with phase modulation where edges that determine the lengths of the certain pits in a track direction are formed at positions whose phases are advanced and delayed by the slight amounts from corresponding positions when the certain pits are formed to record only the primary digital information, and each slight amount may be constant and be within a range of jitter that is observed, during reproduction, for pits on an optical disc on which only the primary digital information is recorded.
Also, the secondary digital information may be recorded with a radial modulation where each certain pit is formed at a position that is displaced by a slight amount from a track center either inward or outward in a radial direction, and each slight amount may be constant and be within a range of position errors in the radial direction that are observed, during reproduction, for pits on an optical disc on which only the primary digital information is recorded.
The optical disc of the present invention into which the secondary digital information has been embedded can be produced by controlling the generation of delays, by a constant delay time, in the channel signal corresponding to the pit string during the jitter modulation. Also, the optical disc of the present invention can be produced by controlling the displacement of the recording head in the radial direction by the slight amount during the radial modulation. This simplifies the construction of a phase modulation circuit for performing the jitter modulation or the radial modulation.
Here, the secondary digital information may be recorded in a certain area of the optical disc, and within the certain area of the optical disc, a total number of edges formed at positions whose phases are advanced may be substantially equal to a total number of edges formed at positions whose phases are delayed. Also, within the certain area, a total number of pits that are formed at positions displaced inward may be substantially equal to a total number of pits that are formed at positions displaced outward.
As a result, in the case of the jitter modulation, the numbers of the advanced phase components and the numbers of the delayed phase components included in the channel signal that is subjected to the phase modulation to record the secondary digital information are substantially equal to each other. As a result, during reproduction, the channel bit clock that is synchronized with the channel signal is extracted with stability, and the sensitivity for detecting the phase modulation components is thereby enhanced. Similarly, in the case of the radial modulation, the number of inner displacements is substantially equal to the number of outer displacements. As a result, during reproduction, a push-pull signal is detected a modulation for with stability, a tracking servo is driven without an offset, and the sensitivity for detecting radial modulation components is thereby enhanced.
Here, within the certain area, advancing phases and a modulation for delaying phases may be performed with equal probability for each unit code of the primary digital information. Also, within the certain area, a modulation for displacing pits inward and a modulation for displacing pits outward may be performed with equal probability for each unit code of the primary digital information.
In the case of the jitter modulation, advanced edges and delayed edges of the channel signal are cyclically detected at short intervals during reproduction. As a result, the channel bit clock is extracted with stability and the secondary digital information is reconstructed with high accuracy. Similarly, in the case of the radial modulation, the channel signal that is displaced toward the inner periphery and the channel signal that is displaced toward the outer periphery are cyclically detected at short intervals during reproduction. As a result, the push-pull signal is detected with stability, the tracking servo is driven without an offset, and the secondary digital information is thereby reconstructed with high accuracy.
The stated object is also achieved by a recording apparatus for recording primary digital information and secondary digital information onto an optical disc by embedding the secondary digital information into the primary digital information. The recording apparatus includes: a primary digital information recording unit for recording the primary digital information by forming pits at discrete reference positions on a track of the optical disc, and secondary digital information recording unit for recording the secondary digital information by displacing positions of certain pits by slight amounts from corresponding reference positions. Here, the secondary digital information recording unit may further include a random number generating unit for generating a random number series, and a logic conversion unit for logically converting the random number series according to the secondary digital information, where the PE modulation unit generates the PE modulation signal according to the logically converted random number series.
With this construction, in the case of the jitter modulation, the edge positions of pits are subjected to the jitter modulation according to the random number series. This makes it difficult to decode the secondary digital information hidden within jitter. Similarly, in the case of the radial modulation, the recording positions of pits are subjected to the radial modulation according to the random number series. This makes it difficult to decode the secondary digital information hidden within jitter.
Here, the secondary digital information recording unit may further include a secondary digital information holding unit for secretly holding the secondary digital information, and the logic conversion unit may sequentially calculate an exclusive OR of a value of each bit composing the secondary digital information in the secondary digital information recording unit and each part having a constant length of the random number series. With this construction, the random number series is not embedded as it is, but a plurality of random number series generated by calculating exclusive-ORs of the random number series and secret information are embedded. This makes it more difficult to decode the secondary digital information.
The stated object is further achieved by a reproducing apparatus for an optical disc that records primary digital information and secondary digital information. Such a reproducing apparatus includes a primary digital information reproducing unit for reproducing the primary digital information by detecting pits formed at discrete reference positions on a track of the optical disc; a clock extracting unit for extracting a channel bit clock that is synchronized with the reference positions from a channel signal corresponding to a string of the detected pits, and a secondary digital information extracting unit for extracting the secondary digital information according to phase differences between the channel signal and the channel bit clock.
The stated object is also achieved by a reproducing apparatus for an optical disc that records primary digital information and secondary digital information. Such a reproducing includes a primary digital information reproducing unit for reproducing the primary digital information by detecting pits that are formed at discrete reference positions on a track of the optical disc, a radial error detecting unit for detecting a displacement of a position of each detected pit from a center of the track in a radial direction, and a secondary digital information extracting unit for extracting the secondary digital information according to the detected displacements.
Here, the reproducing apparatus may further include a reproduction restricting unit for restricting a reproduction of the primary digital information if the secondary digital information generating unit determines that each integral that is calculated by the integrating unit does not exceed the predetermined threshold value.
Also, the secondary digital information generating unit may determine whether a first correlation, a second correlation, or no correlation exists for each integral that is calculated by the integrating unit and, if either the first correlation or second correlation exists for each integral, the secondary digital generation unit outputs the existing correlations as the secondary digital information, where the first correlation means that an integral is greater than a predetermined position threshold value, and the second correlation means that an integral is smaller than a predetermined negative threshold value. The reproducing apparatus may further include a secret information storing unit for storing secret information, and a verification unit for comparing the secret information and the secondary digital information that is outputted from the secondary digital information generating unit, where the reproduction restricting unit may also restrict the reproduction of the primary digital information if the verification unit determines that the secret information does not match the secondary digital information.
This realizes an optical disc reproducing apparatus having a function of protecting copyrights by reading the secondary digital information that is embedded with the jitter modulation and the radial modulation.
Here, the primary digital information may be an information pit string that is formed in synchronization with a predetermined reference clock, and the secondary digital information may be copyguard-information represented by a pit string that is formed by locally adding phase errors to the predetermined reference clock.
With this construction, during recording, the reference clock is subjected to a phase modulation in response to the copyguard information, and the data string is recorded according to the phase-modulated clock. As a result, the duplication of the optical disc is difficult. During reproduction, the copyguard information is detected by detecting jitter values as phase shift components. This makes it possible to identify illegally duplicated optical discs by using an apparatus having a simple circuit construction.
The present invention may be realized as a recording method and a reproducing method that include steps for achieving the functions of the characteristic construction elements of the optical disc recording apparatus and the optical disc reproducing apparatus.
With the jitter modulation and the radial modulation of the present invention, illegal activities caused by dubbing optical discs storing digital contents are prevented, and a secure distribution of the digital contents is realized. Therefore, the present invention has a great practical use.